Understanding Enthalpy of Neutralisation
Dot-Point 5: conduct a practical investigation to measure the enthalpy of neutralisation (ACSCH093)
The enthalpy of neutralisation refers to the heat released or absorbed during the neutralisation of an acid and a base. This is generally an exothermic process, meaning energy is released as the acid and base react to form water and salt. The heat released can be measured experimentally and used to calculate the molar enthalpy change of the neutralisation reaction.
Chemical Equation for a Neutralisation Reaction:
A general neutralisation reaction between an acid and a base is:
H⁺ (aq) + OH⁻ (aq) → H₂O (l)
For example, the neutralisation of hydrochloric acid (HCl) with sodium hydroxide (NaOH) is:
HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)
This reaction produces water and salt, releasing energy in the process.
Hypothesis
If the total heat released from the neutralisation of a strong acid (HCl) and a strong base (NaOH) can be measured, then the molar enthalpy and standard enthalpy of neutralisation can be calculated.
Procedure
Measure the Mass of the Calorimeter:
Record the mass of the calorimeter with its contents before starting the experiment.
Set Up the Calorimeter:
Place the temperature probe, stirring magnet, and magnet stirrer into the calorimeter.
Measure Solutions:
Use a measuring cylinder to measure 50 mL of diluted 1M HCl and 50 mL of diluted 1M NaOH solutions.
Add Acid to the Calorimeter:
Turn on the magnet stirrer and add the HCl into the calorimeter, ensuring that the temperature probe is submerged in the acid solution.
Record Initial Temperature:
Record the initial temperature of the acid for three minutes, allowing the temperature to stabilise (minimal change in temperature).
Add Base to the Calorimeter:
Quickly pour all of the NaOH solution into the calorimeter containing HCl.
Record Temperature Change:
Immediately record the temperature for three minutes after the base has been added. You should observe a sharp rise in temperature as the neutralisation reaction occurs. This is the heat released during the exothermic neutralisation.
Final Temperature:
The final temperature should be recorded as the maximum temperature reached, as heat loss will prevent the system from reaching its theoretical maximum.
Calculate Molar Heat of Neutralisation (ΔHₘₒₗ):
Use the following formula to calculate the heat released during the neutralisation:
Heat of Neutralisation Calculations:
Formula for Heat Released (q):
To calculate the heat released, use the equation:
q = m × C × ΔT
where:
q = Total heat released (J or kJ)
m = Mass of the solution (kg) — for this, combine the masses of acid and base, assuming the densities of the solutions are close to that of water (1 g/mL).
C = Specific heat capacity of water (4.18 kJ/kg/K or 4180 J/kg/K)
ΔT = Change in temperature (final temperature – initial temperature in K)
For example, if the combined mass of the solution is 0.1 kg (100 g), and the change in temperature (ΔT) is 5°C (5 K), the heat released (q) would be calculated as:
q = 0.1 kg × 4.18 kJ/kg/K × 5 K
q = 2.09 kJ
where:
q = Total heat released (J or kJ)
m = Mass of the solution (kg) — for this, combine the masses of acid and base, assuming the densities of the solutions are close to that of water (1 g/mL).
C = Specific heat capacity of water (4.18 kJ/kg/K or 4180 J/kg/K)
ΔT = Change in temperature (final temperature – initial temperature in K)
Molar Heat of Neutralisation (ΔH neutralisation):
Once you know the total heat released, you can calculate the molar heat of neutralisation (ΔH neutralisation) by dividing the total heat (q) by the number of moles of the acid or base that reacted:
ΔH neutralisation = -q / n
Where:
q = Total heat released (J or kJ)
n = Moles of acid or base that reacted (mol)
For example, if 0.05 moles of HCl reacted (50 mL of 1M HCl), the molar heat of neutralisation would be:
ΔH neutralisation = -2.09 kJ / 0.05 mol = -41.8 kJ/mol
Standard Enthalpy of Neutralisation (ΔH° neutralisation) and Calculation for HCl:
The standard enthalpy of neutralisation is the enthalpy change for the neutralisation of a strong acid with a strong base under standard conditions (298 K, 1 atm pressure). It is often measured as the heat released when 1 mole of water is formed in the neutralisation.
For a typical strong acid (HCl) and strong base (NaOH):
HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)
The standard enthalpy of neutralisation is approximately -57.3 kJ/mol for the formation of 1 mole of water.
To calculate this, we can divide the heat released by the moles of water produced. If 1 mole of water is produced in this reaction, the molar enthalpy change would be:
ΔH° neutralisation = -41.8 kJ/mol
Important Notes
Ensure matching units:
Heat should be in Joules (J) or Kilojoules (kJ).
Mass should be in kilograms (kg).
Temperature in Kelvin (K).
Moles should be calculated from molarity (mol/L) and volume (L).
For accurate results, ensure minimal heat loss during the experiment and proper calibration of temperature probes.
By conducting this practical investigation, you can measure the heat released during the neutralisation of HCl with NaOH and use the data to calculate the molar enthalpy and standard enthalpy of neutralisation.
